1) Field of the Invention
The present invention relates to a digital-to-analog converter which converts a digital signal into an analog current by using a plurality of current cells.
2) Description of the Related Art
Current cell type digital-to-analog converters are known in the art. Such digital-to-analog converter has a capability to convert a digital signal into an analog current by using a plurality of current cells. The current cell type digital-to-analog converter is disclosed in, for example, Japanese Patent Kokai No.11-17545.
When, for example, the current cell type digital-to-analog converter has a 6-bit resolution, the current values having 64 levels from 0 level to 26−1 level are usually set up. In order to set up such levels, the digital-to-analog converter having the 6-bit resolution includes 63 current cells. Accordingly, when the current cells are turned on in such a manner that the number of turned-on current cells corresponds to a digital value, an electrical current corresponding to such digital value can be output.
The current cell type digital-to-analog converter disclosed in the above mentioned Japanese Patent Kokai No.11-17545 includes 63 current cells weighted by 4, one current cell weighted by 2, one current cell weighted by 1, one current cell weighted by ½ and one current cell weighted by ¼ (see FIG. 1 of Japanese Patent Kokai No.11-17545). The current cells weighted by 4 are arranged in a matrix pattern. On the other hand, the other weighted current cells are not arranged in a matrix pattern, because only one cell is assigned for each weight. Each current cell is connected to a power supply line VCC (see FIG. 2 of Japanese Patent Kokai No.11-175453). Each current cell outputs an electrical current supplied from the power supply line VCC when the cell is selected by a decoder. When the electrical current output from the current cell weighted by 1 is Io, the electrical current output from the current cell weighted by 4 is 4Io, the electrical current output from the current cell weighted by 2 is 2Io, the electrical current output from the current cell weighted by ½ is Io/2, and the electrical current output from the current cell weighted by ¼ is Io/4. Combining the current cells which are weighted differently with respect to each other provides an increased resolution in the digital-to-analog converter without using a number of current cells.
It is difficult to sufficiently expand the width of the power supply line when a number of current cells are provided in one digital-to-analog converter, because of an area restriction and other restrictions. Accordingly, when a large number of current cells are connected to one power supply line, an influence of a voltage drop due to a resistance of the power supply line can not be ignored.
It is preferable for the digital-to-analog converter that the output current increases Io by Io upon an increment of the digital value by 1. As mentioned earlier, Io represents the electrical current output from the current cell weighted by 1. However, such increments of the electrical current usually fluctuate in a practical situation due to fluctuation in the output currents of the current cells. Therefore, a conventional current cell type digital-to-analog converter has a drawback of bad differential nonlinearity.